There are known prior-art processes for the production of expandable or expanded thermoplastic polymers.
U.S. Pat. No. 6,342,540 B1 discloses a process for the production of expandable styrene polymers comprising water as blowing agent. For this, a polystyrene melt is mixed with water as blowing agent in the presence of an emulsifying aid in an extruder, and is extruded. The conduct of the process is complicated.
EP 1 347 008 A2 discloses a process for the production of expanded items of biodegradable plastics material, comprising the mixing of the plastics material in the molten state with water in an extruder, extrusion of the mixture, and cooling of the resultant extrudates.
WO 2007/030719 A2 discloses a process for the production of a polystyrene foam comprising a nanoclay free from modifiers. To produce said polystyrene foam, a mixture of water and of the clay mentioned is emulsified in a mixture composed of prepolymerized polystyrene and styrene, and this is followed by the polymerization of the prepolymer. The polystyrene foams are then produced via separation of the water present in the polymer.
WO 2008/087078 A1 discloses a hybrid system composed of foamed thermoplastic elastomers and polyurethanes, and also a process for its production. From this, the thermoplastic polyurethane in the form of granules is heated in a sealed reactor to above the softening point with water, with a suspending agent, and with a blowing agent. According to WO 2008/087078, organic liquids or inorganic gases, or a mixture of these, is/are used as blowing agent.
WO 2007/082838 A1 discloses foams based on thermoplastic polyurethanes, and also a process for their production. According to said document, the expanded thermoplastic polyurethane particles are produced via suspension or extrusion processes. In the case of the suspension process, the thermoplastic polyurethane in the form of granules is heated in a sealed reactor to above the softening point of the granules, with water, with a suspending agent, and with the blowing agent. The polymer particles here are impregnated with the blowing agent. Steam or hot air is used to foam said expandable particles.
In the process known from the prior art, a blowing agent is used for the foaming of the corresponding thermoplastic polymers, and suitable blowing agents here according to the prior art are organic liquids or organic or inorganic gases, examples being hydrocarbons, and gases such as nitrogen, carbon dioxide, etc. A disadvantage of said blowing agents is that they are toxic and/or inflammable. A further disadvantage of gaseous blowing agents is that the conduct of the process is complicated.